H. Graener

Ultrafast dynamics of hydrogen bonds directly observed by time‐resolved infrared spectroscopy

Internal, hydrogen bonded OH groups of ethanol oligomers (solvent CCl4) are vibrationally excited by intense picosecond pulses at 3320 cm−1. The transient band shape observed in the OH stretching region (3000 to 3700 cm−1) is monitored by an independently tunable picosecond infrared pulse. The bands in this region are direct probes of hydrogen bridges. The time dependent growth and decay of these bands provides strong evidence for rapid bond breaking with a vibrational predissociation time of ≊5 ps, and for partial reassociation with a time constant of ≊20 ps.

Ultrashort laser pulse induced deformation of silver nanoparticles in glass

Irradiating intense femtosecond laser pulses on a glass sample containing silver nanoparticles results in permanent sample color changes if the laser wavelength is in the region of the particles’ surface plasmon resonance. In particular, even a single pulse of appropriate intensity can modify the initially isotropic extinction of glass containing spherical particles to a dichroic sample behavior in the irradiated area. This observation is interpreted as ultrafast particle deformation induced by the laser pulse creating nonspherical particles of uniform orientation.

Sum-frequency vibrational spectroscopy at the liquid—air interface of methanol. Water solutions

The vibrational spectrum of surface methanol molecules is investigated in the CH3 stretching region in a large concentration range corresponding to CH3OH mole fractions of the bulk of 0.011 ⩽x⩽ 1. Striking spectral changes are observed for the water-methanol surface network with excess number densities of methanol and increasing molecular ordering upon dilution. For neat methanol (x=1 ) the methyl groups are found to be oriented with an average tilt angle θ0<40° of the symmetry axis with respect to the surface normal. For the diluted system, x ⩽ 0.1, our data suggest that the molecules are polar oriented with θ0 < 16°.

Ultrafast vibrational predissociation of hydrogen bonds: Mode selective infrared photochemistry in liquids

Using intense ultrashort excitation pulses in the infrared (IR) tuned to the CH or oligomeric OH absorption bands of ethanol dissolved in CCl4, vibrational predissociation and partial reassociation of hydrogen bonds are observed on the picosecond time scale. Induced absorption at the OH stretching frequency 3500 cm−1 of proton donor end groups serves as a spectroscopic probe of the broken H bridges and is studied as a function of time by the help of delayed, tunable interrogation pulses. Different values for the effective dissociation rates and total quantum yields provide strong evidence for mode specifity of the IR photodissociation in the liquid at ambient temperature. The experimental results on vibrational population decay and energy transfer rates suggest a V–V transfer mechanism for CH to OH modes for the vibrational predissociation after CH excitation.

Vibrational and orientational relaxation of monomeric water molecules in liquids

A polarization resolved infrared double resonance experiment with picosecond pulses has been used to measure the vibrational and orientational relaxation times of monomeric water and heavy water molecules in different organic solvents after excitation of the antisymmetric ν3 vibration. In all investigated systems a fast (but definitely time resolved) equilibration among the OH (OD) stretch vibrations was found, followed by a rather slow decay of this ensemble. Furthermore hints to a medium lived intermediate state, most likely the bending vibration, are reported. The principal relaxation scheme is very similar to that of water vapor. Comparing H2O and D2O in different solvents significant differences were found, which can be understood at least semiquantitatively. The orientational relaxation times of H2O and D2O differ by a factor of 2, which can be explained by the influence of weak hydrogen bonds of different strength for H2O and D2O on the orientational relaxation. Additionally interesting aspects of ...

Broadly tunable femtosecond pulses generated by optical parametric oscillation

A singly resonant optical parametric oscillator is demonstrated that uses a beta-barium borate crystal and is synchronously pumped by microsecond pulse trains of a Nd:glass laser (0.8 psec, 527 nm). Broadly tunable pulses are generated in the wavelength range 0.7-1.8 microm with a duration of 160-260 fsec and an energy conversion of 3%. Steep pulse wings are observed, with a 1/e decay time of approximately 90 fsec. Under special conditions, parametric pulses as short as 65 +/- 7 fsec are obtained.

Direct observation of rotational relaxation times by time-resolved infrared spectroscopy

Abstract Using picosecond pulses tuned to vibrational absorption bands, time-resolved polarization spectroscopy is extended to the infrared and to the study of the orientational motion of small molecules in condensed matter. Experimental data are presented for CHBr3 in different solvents.

Poling-assisted bleaching of metal-doped nanocomposite glass

Thermal poling of soda-lime glass which was doped with spherical or ellipsoidal silver nanoparticles has revealed what we believe to be a phenomenon of general interest in the physics of nanocomposite materials: The field-assisted dissolution of metal nanoparticles embedded in glass. Macroscopically, this phenomenon manifested itself as poling-assisted bleaching of the glass in the sense that the glass became more (or even completely) transparent under the anode. The phenomenon is physically interpreted in terms of the ionization of metal nanoclusters followed by the removal of ions from the clusters and their drift in the depth, under the action of the extremely high electric field which is created underneath the anodic surface during poling. The underlying physical mechanism is expected to offer unique opportunities for the control of structural and optical properties of nanocomposite glasses.

Vibrational and reorientational dynamics of water molecules in liquid matrices

Abstract Polarization-resolved double-resonance spectroscopy with tunable picosecond pulses in the infrared is used to study the vibrational and reorientational relaxation of monomeric water molecules in various organic solvents. After excitation of the antisymmetric OH stretching vibration ν 3 a rapid population redistribution with the neighboring stretching vibration ν 1 is observed, followed by a much slower and strongly solvent-dependent population decay of this vibrational ensemble. The findings are explained by energy transfer to the overtone 2ν 2 of the bending mode and the influence of weak hydrogen bonds between water and solvent molecules. The latter govern the energy difference between the initial (ν 1 ) and final (2ν 2 ) state of the relaxation process. A theoretical discussion of polarization effects is presented taking into account the varying directions of the transition dipole moments of different modes in the molecular coordinate frame. It is shown that measurements of the induced dichroism facilitate the interpretation of our transient infrared spectra and allow the determination of reorientational relaxation times. The reorientational time constants also strongly depend on the weak hydrogen bonds between solute and solvent molecules.

Infrared double-resonance spectroscopy of bromoform with picosecond pulses

Abstract Using independently tunable pump and probe pulses in the infrared, time- and frequency-resolved spectroscopy of vibrationally excited, polyatomic molecules in liquids is demonstrated for the first time. Experimental data are presented for CHBr 3 , measuring the population lifetime via excited-state absorption of the CH-stretching mode; for larger probe delay, the non-equilibrium population of intermediate vibrational levels in the relaxation ladder of bromoform is observed.